In closed-loop MIMO systems, channel state information is transmitted from a receiver to a transmitter. The transmitter may then use the transmit channel state information for beam forming, i.e. for combining or separating spatial streams of a transmitted signal in a multiple transmit antenna system. However, communications bandwidth is consumed to transmit current channel state information, reducing overall data throughput. It is desirable to reduce the bandwidth used for feedback of channel state information.
In general, for a system with a plurality of N transmit and N receive antennas, the current channel state information may be represented by a N×N unitary beamforming matrix V, determined using a singular value decomposition (SVD) algorithm. It is required that the receiver transmits information for each entry of the beamforming matrix back to the transmitter. Transmitting this channel state information consumes bandwidth that might otherwise be used for data traffic.
Precoding is a closed-loop technique that is used to increase the spectrum efficiency of MIMO wireless networks. The optimum precoder for the frequency flat channel of a point-to-point MIMO link is the right unitary matrix of the singular value decomposition (SVD) outputs of the channel matrix. For example, in cyclic single carrier systems, precoding can be performed in the frequency domain. The receiver can estimate the frequency domain channel matrices, perform SVD for the channel matrix on each subcarrier, then feed the unitary precoder of each subcarrier back to the transmitter. Various precoding schemes are known for reducing the feedback overhead.
Known approaches are disclosed, for example, in U.S. Pat. No. 8,165,241 B2, issued on Apr. 24, 2012, to Lin et al., entitled “Closed Loop Feedback in MIMO Systems”; 2. U.S. Pat. No. 7,961,802 B2, issued Jun. 14, 2011, to Li et al., entitled “Interpolation in Channel State Feedback”; and 3. U.S. Pat. No. 8,320,283 B2, issued on Nov. 27, 2012, to Kim, entitled “Method and System for Utilizing Givens Rotation Expressions for Asymmetric Beamforming Matrices in Explict Feedback Information. For a two transmit antenna scenario, the precoding schemes disclosed in these patents use two angles to represent the precoder and perform interpolation using an angular representation factoring out phase information (Lin), a Householder representation to reduce the dimensionality of the matrix (Li), or a Givens rotation representation to reduce the number of bits required to encode the unrotated matrix (Kim), for a two transmit antenna scenario.
A well-known LTE approach uses a codebook per subcarrier. An article by N. Khaled et al., entitled “Interpolation-based multi-mode precoding for MIMO-OFDM systems”, Proc. EUSIPCO 2005, discloses linear interpolation of the entire matrix and normalization afterwards.
The present invention seeks to provide an improved or alternative method for reducing feedback overhead within fixed wireless backhaul networks comprising MIMO closed-loop systems, with particular application to two antenna systems.